1. Field of the Invention
The present invention relates to a wheel braking system for an automotive vehicle, and more particularly to a wheel braking system, which is easy to tune to meet various demands.
2. Description of the Related Art
U.S. Pat. No. 5,080,447, which corresponds to JP-A 4-230462, discloses a wheel brake unit operated by hydraulic pressure provided by a master cylinder and a hydraulic boost unit operated by the vehicle operator. The hydraulic fluid under pressure from the master cylinder is provided to the brake unit via brake lines and a pressure generator. An electronic controller controls the pressure generator to limit the brake pressure applied to the brake unit to prevent wheel lockup. When the controller senses a braking condition whereat the wheel is approaching an incipient wheel lock, the pressure generator is controlled to regulate the braking pressure to the wheel to maintain the braking of the wheel in a stable braking region.
The pressure generator includes a DC torque motor whose output shaft drives a gear train that, in turn, rotates a linear ball screw actuator. The ball screw actuator contains a linearly stationary ball screw that, when rotated, linearly positions a nut. The nut terminates in a position such that as the linear ball screw rotates, the piston is either extended or retracted depending upon the direction of rotation of the torque motor. The generator includes a housing in which a cylinder is formed. The piston is reciprocally received within the cylinder. The cylinder forms a portion of the fluid path between the master cylinder and the wheel brake unit. Included within this fluid path is a normally closed ball check valve, which, when closed, isolates the master cylinder from the wheel brake unit. The ball check valve is operated to an open position by the piston when it is positioned in an extended position within the cylinder. This position is the home position during normal vehicle braking. When the ball check valve is opened, fluid communication is provided between the master cylinder and the wheel brake unit. This position is the normal inactive position of the pressure generator so that the vehicle operator provides normal braking of the wheel of the vehicle upon actuation of the brakes. However, when the controller operates the torque motor to modulate the braking pressure in the wheel brake unit, the piston is retracted. This allows the ball check valve to seat and isolate the master cylinder from the wheel brake unit as long as the pressure in the cylinder is less than the pressure from the master cylinder. Further retraction of the piston functions to increase the volume in the cylinder, thereby decreasing the pressure applied to the wheel brake unit. By controlling the DC torque motor, pressure at the wheel brake can be modulated to controlled values less than the master cylinder pressure output. This exists until such time that the piston again unseats the ball check valve or until the pressure generated by the pressure generator at the wheel brake unit exceeds the fluid pressure output of the master cylinder. When this latter condition exists, the ball check valve is opened by the differential fluid pressure, which limits the pressure of the wheel brake unit at the master cylinder.
The known braking system employs the hydraulic boost unit for operation during vehicle normal braking and the pressure generator for operation during anti-lock braking. As mentioned before, the pressure generator includes the DC torque motor, gear train, linear ball screw actuator, and ball check valve. It would be desirable to reduce the number of component parts for reduced manufacturing cost. As rotation of the DC torque motor is converted into reciprocal motion of the piston via the gear train and linear ball screw actuator, the controller employs sophisticated and complicated control logic. This also pushes up manufacturing cost. Thus, it would be desirable to seek simplicity in the control logic employed by the controller. In this known system, the wheel cylinder pressure can never exceed the operator-established pressure. Thus, the hydraulic boost unit is needed in the known braking system according U.S. Pat. No. 5,080,447 to assist vehicle operator brake pedal effort.
JP-A 5-147524 discloses a hydraulic braking system in which the hydraulic fluid under pressure from a master cylinder is provided to a wheel brake unit via a gear pump that is driven by a DC motor. An electronic controller controls the gear pump to regulate supply of hydraulic fluid under pressure to and discharge thereof from the wheel brake unit to prevent wheel lookup. When the controller senses a braking condition whereat the wheel is approaching an incipient wheel lock, the DC motor is activated to turn the pump in a direction to discharge hydraulic fluid from the wheel brake unit. Subsequently, when the wheel is escaping from the incipient wheel lock, the controller reduces current passing through the DC motor, allowing the DC motor to turn in the opposite direction to increase the braking pressure applied to the wheel brake unit. When the controller supplies no current to the DC motor, the hydraulic fluid under pressure from the master cylinder pushes the gears of the pump open to reach the wheel brake unit. Thus, fluid path is provided between the master cylinder and the wheel brake unit when the vehicle operator operates the master cylinder.
According to this known braking system, the wheel brake pressure is determined as a function of the pump speed and the vehicle operator brake effort such that, with the same brake effort, only one wheel brake pressure value is established for one pump speed value. It is desirable for tuning the braking system to meet different demands that different wheel brake pressure values be established for one pump speed value, if need arises.
The present invention aims at providing a solution to this problem in the hydraulic braking system of this kind.